Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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Z . Dajic
2000). In Arabidopsis, three of nine, so far identified genes of the Shaker K + channel
family: AKT1, AKT2 and SKOR, encode proteins that regulate K + uptake by the root, K +
transport in the phloem tissues and K + secretion into the xylem sap, respectively, whereas
a novel gene AtKC1 is included in K + uptake from the medium (Pilot et al., 2003).
Plasmallemma voltage-insensitive cation channels (VICs) and K + in channels
are responsible for an influx of sodium across the plasma membrane (White, 1999). The
VIC channels are non-selective amongst monovalent and, in same cases, divalent cations
and their role is related to low affinity Na + uptake and the stabilization of membrane
potential, as well as the fast adaptation to osmotic stress (Maathuis and Amtmann,
1999). The VIC channels exhibit lower selectivity for K + over Na + than rectifying channels,
thus providing the massive Na + entry in saline conditions over a wide range of
voltages (Amtmann and Sanders, 1999).
8.2.3. Determinants of Anion Transport
Anion uptake is opposed by negative internal membrane potential, and it must be
accompanied by the proton uptake to be energetically active (Michelet and Boutry,
1995). Influx of Na + facilitate uptake of Cl - down the chemical gradient. Chloride is
thought to traverse the root by a symplastic pathway. Anion channels regulate anion
efflux from the cell through plasmalemma and the tonoplast (Krol and Trebacz, 2000).
Electrophysiological and biochemical studies demonstrated the presence of an electrogenic
symporter mediating the Cl - influx and efflux across the plasma membrane, and the
Cl - antiporters are involved in the chloride transport into the vacuole (White and
Broadley, 2001).
8.2.4. Determinants of Sodium Compartmentation
Accumulation of sodium in the vacuole is dependent on vacuolar H + -translocating
enzymes and tonoplast Na + /H + antiporters, which are induced by saline environment
(Barkla and Pantoja, 1996). An immediate effect of salt stress is vacuolar alkalization,
linked with Na + /H + antiporter activity of tonoplast vesicles (Hasegawa et al., 2000). The
first plant Na + /H + antiporter gene, exhibiting high homology with yeast antiporter NHX1,
was isolated from Arabidopsis and designated as AtNHX1 (Xiong and Zhu, 2002).
Research studies of Arabidopsis antiporters confirmed their role in salinity tolerance of
plants (Aharon et al., 2003).
The Arabidopsis AtNHX family comprises six genes, indicating that plants
have significant need to regulate Na + homeostasis through vacuolar compartmentation,
independently on the fact that sodium is not an essential mineral element (Yokoi et al.,
2002b). Functional characterization of AtNHX members showed that AtNHX1 and
AtNHX2 transcripts were widely distributed in all tissues (in difference to low abundance
of AtNHX5 transcripts), whereas AtNHX3, and AtNHX4 transcripts were found
in the flower and root tissues, respectively (Aharon et al., 2003). AtNHX2 and AtNHX5,